If you leave your car behind and join a ranger-led hike in Southwest Colorado's Mesa Verde National Park, you'll find yourself at a spot where the scrubby pinyon-juniper forest drops off into a sandstone chasm. It reveals a maze of 800-year-old stone dwellings wedged beneath an overhang in the canyon wall. They're so well preserved that it's easy to imagine you've stepped back in time. And that nothing has changed in this high desert landscape since the Ancestral Puebloans built these chambers in the 12th century.

But there's a modern problem plaguing Mesa Verde and dozens of other national parks. It is air pollution. Mesa Verde lies downwind of several coal-fired power plants. They release nitrogen, mercury and sulfur into the air. Huge natural gas fields lurk to the south, belching methane. And as nearby towns and cities grow, everyday activities like driving increase levels of harmful ozone. Hundreds of years ago, Ancestral Puebloans would have been able to look out from Mesa Verde and see views that stretched 170 miles. Today, haze reduces those views to just 66 miles on the worst days.

"Air pollution knows no boundaries," says Ulla Reeves. She is Clean Air Campaign Manager with the National Parks Conservation Association. It is a nonprofit that advocates on behalf of parks. "It reaches many, many miles away from the source." In Mesa Verde, one of the sources of pollution is Las Vegas. That city is 500 miles away.

In an analysis last year, the NPCA found that even parks with the most protection under the Clear Air Act -- icons like Mesa Verde, Everglades, Yosemite, Acadia and Sequoia -- continue to experience pollution. It can affect wildlife and human health, as well as the climate. According to the National Park Service's data, ozone levels on the peaks of the Great Smoky Mountains, for example, are nearly twice those in nearby cities like Atlanta. Up to 90 percent of black cherry trees in the park (depending on location) have sickly yellow leaves and other signs of ozone damage. Visitors with asthma can have trouble breathing. In California, Sequoia and Kings Canyon national parks regularly have ozone pollution that exceeds the 70 parts per billion standard set by the Environmental Protection Agency.

The federal government has long recognized that air pollution doesn't stop at park borders. In 1999, the EPA created a regulation called the Regional Haze Rule. It is designed to return visibility in 156 national parks and wilderness areas back to "natural" conditions. The plan is to cut emissions from polluters like coal-fired power plants. The rule only tackles visibility. But "the pollutants that affect visibility can also affect ecosystems and human health," says John Vimont. He is chief of the research and monitoring branch of the National Park Service's Air Resources Division.

The rule has played an important role in getting some facilities to adopt cleaner technologies. Over the last 10 years, average visibility in Great Smoky Mountains National Park has risen from 20 miles to 46 miles, says Reeves. But there's still a long way to go. Visibility in the Great Smoky Mountains should be 112 miles on the best days. Part of the reason for the slow progress is because the rule is largely interpreted and carried out at the state level, rather than by federal agencies. Many states have struggled to muster resources and meet deadlines.

That's why the EPA is currently working on a series of changes. They are meant to strengthen the Regional Haze Rule. The changes will force states to keep more robust data on their progress. The states must submit regular plans to ensure they're meeting legal requirements and cutting emissions. At the same time, the changes allow states even more time to implement their next round of plans.

Even if the Regional Haze Rule is strengthened, though, it'll still take a long time for the air in national parks to return to pre-industrial quality. Under standards imposed a decade ago, the NPCA estimates that the soonest that goal could be achieved is the year 2064. Thirty out of 157 national parks are predicted to return to natural conditions by that year. Others, like Arizona's Saguaro National Park, might take much longer. It could be 750 years. Again, these dates don't take into account the latest changes, which could speed up recovery time. But they're still a sobering reminder that even in some of the most protected landscapes on the planet, the effects of human activity can linger well beyond our own lifetimes.

In Mesa Verde, natural resource manager George San Miguel is keenly aware of the effect that air pollution has on the park's visitors. Airborne nitrogen and sulfur is deposited into the soil, which leads to more invasive weeds and fewer native grasses. Methane hovering overhead accelerates climate change. And then, of course, there are the views.

"One of the things we try to instill in visitors is a sense of going back in time," San Miguel says. "We want visitors to immerse themselves in the past; to put themselves in the sandals of the Native Americans that lived here, so to speak." To do that, he explains, you need to be able to see a long distance, because the Ancestral Puebloans likely used distant desert towers as navigation aids. Until Mesa Verde's natural visibility is restored, visitors remain solidly planted in the 21st century.

I hope that pollution in general would start to decrease. More pollution means more harm to the environment. It could harm and endanger wildlife animals. People sometimes throw trash in the oceans and forests with a lot of germs. The wildlife sometimes eat it and gets hurt by it. Some organizations are trying to stop this problem and make the air more cleaner. I thought that all parks would have clean and fresh air because it is natural and out in the open but it is not so. The harmful gases that are emitted through factories hurt the ozone layer. It is difficult to stop air pollution in national parks because of power plants. There are also gases like nitrogen that hover around and harm the park. There are also harmful chemicals that go into the dirt and harm the ecosystem.

ochristina-dav

8/24/2016 - 04:27 p.m.

In response to "Even in "pristine" national parks, the air's not clear", I agree that air pollution is a major problem for parks and other national places. One reason I agree is that air pollution effects the wildlife in the park, leading to visitors not wanting to come back to visit the national area. Another reason is that pollution can spread many miles and cover a whole park, leading to the first reason I stated. It even says in the article "'Air pollution knows no boundaries,' says Ulla Reeves." A third reason is that can effect the visitors coming there and make them sick or harm them, just like pollution does to the parks. Pollution is technically littering the environment. Even though most people do not believe others should worry about pollution coming to national parks, I think that it is a problem to both the environment and visitors.

ybailey-dav

8/25/2016 - 10:05 p.m.

In response to "Even in 'pristine' national parks, the air's not clear," I agree that air pollution should end. One reason I agree is that the ozone layer is being damaged, because of that 90% of the black berry trees in that park are being damaged and getting signs that the ozone layer is getting damaged like leafs are turning a sickly yellow color. Also because of the damage to the trees people who have asthma can have difficulty in breathing. Another reason is that the wonderful views you used to could see cant see anymore because of air pollution. It says in the article " Hundreds of years ago, Ancestral Puebloans would have been able to look out from Mesa Verde and see views that stretched 170 miles. today, haze reduces those views to just 66 miles on the worst day." A third reason I agree is that all the animals, plants, and water is being effected by this too. Animals and plants breath the same oxygen that we do. Even though most people don't think about how they're polluting the air, I think it should be taken care of.

maryjanem-eat

8/28/2016 - 04:40 p.m.

They should be working more on how to get the pollution down and to make things that wont pollute the earth as they already have. Its pretty sad about how much the human race has polluted the air and still does it and some don't even care. They should more things run on solar energy than on gas and oil. Hopefully they can figure how to get the pollution down to a safe level as soon as possible because a lot of people are going to get really sick and will be harmed by the pollution. People should start to pay more attention to what they are doing to the earth and the human population when they are polluting the earth. Hopefully they can figure out how to make the pollution back down to safe levels before to long.

cmaizie-dav

9/08/2016 - 07:47 p.m.

In response to "Even in "pristine" national the air is not clear ," I agree that it is difficult to stop air pollution. One reason I agree is that the Regional Haze Rule is not regulated by the federal government, but instead by the states. This makes it harder for the same laws to be enforced in different states. Another reason is that the effects are caused by daily things we do. It says in the article that thirty out of the 157 national parks will return to natural conditions by the year 2064..